1. Field of the Invention
The present invention relates to a signal processing method, a signal output apparatus, a signal processing apparatus, an image processing apparatus and an image forming apparatus in which an arbitrary frequency characteristic can be set and a noise signal is generated with a comparatively low resource.
2. Description of Related Art
Conventionally, for an image output apparatus such as a digital color copying machine or an ink jet copying machine, there has been proposed an image signal processing apparatus in which image quality is designed taking a visual characteristic of a human being into consideration and an input image signal is processed in such a manner as to acquire a visually preferable image (see, for example, Japanese Patent No. 2894117). In the image signal processing apparatus described in Japanese Patent No. 2894117, a noise signal of a spatial frequency response difficult to visually recognize is superposed on an input image signal read from an original, so that image noises and gray level difference existing in the original are reduced without adversely affecting image quality characteristics such as sharpness and color.
In the above-described image signal processing apparatus, the noise signal is generated utilizing a two-dimensional Fourier transform or a inverse Fourier transform in order to give a required frequency characteristic to a noise signal to be superposed. Generally, in this two-dimensional Fourier transform, two-dimensional data is divided into rows and columns, and after executing a one-dimensional Fourier transform for each row, the one-dimensional Fourier transform is executed for each column, thereby obtaining a desired result. This method requires a work memory of the same capacity as the amount of the required noise data. In addition, the two-dimensional Fourier transform requires an additional processing time in response to the amount of the required noise data. In the case where a vast amount of noise data is required, therefore, there arises a problem that the noise data cannot be obtained before the end of the process. In order to avoid this disadvantage, there has been proposed a method for storing the noise data calculated in advance in a memory and using the noise data. However, this method has a problem that the memory for storing the noise data requires as much capacity as the amount of the required noise data.